The present invention relates to the Ocean Thermal Energy Conversion (“OTEC”) system and more particularly to the Cold Water Pipe (“CWP”) for such system. Since the present invention is also applicable to the Surface Water Pipe (“SWP”) and the Discharge Water Pipe (“DWP”), if needed for the system, the three pipes will be herein called individually the Ocean Thermal Energy Conversion Water Pipe (“OTECWP”) or, if reference is made to more than one, as Ocean Thermal Energy Conversion Water Pipes (“OTECWPs”). OTEC systems are energy (and fresh water) producing systems that exploit the temperature difference between warm surface waters in tropical seas and the cold waters in deeper ocean strata. Typically an OTEC system would include a plant mounted on a platform, a ship or barge and a large diameter CWP. Cold water at depths of about 1,000 m is pumped to the surface thorough the CWP, directing the cold water into a power module. The power module also receives warm water from the surface. The temperature differential between the cold water and the warm water is then exploited in the generation of electric energy through well known OTEC techniques.
A conventional Closed Cycle OTEC system will be discussed in detail. A working fluid, which is contained within the closed cycle, is pumped by a liquid pump into an evaporator, where heat from a warm water intake is transferred from the warm water to the working fluid to generate a working fluid vapor. The warm water exiting the evaporator is discharged to the sea. The working fluid vapor enters a turbo-generator to generate electricity by conventional techniques. The working fluid vapor exits the turbo-generator and is condensed in a condenser utilizing cold sea water as a heat sink. The condensed working fluid is then fed back to the liquid pump in order to complete the closed system. Additional information of the OTEC technology can be found in “Renewable Energy from the Ocean—A Guide to OTEC” by Avery and Wu—Oxford University Press—1994.
Water temperature at depths of about 1,000 m is about 4° C., while surface water in the tropics is about 25-28° C. The energy available from such small temperature difference is little, requiring moving large quantities of both deep and surface water. As an illustration, to produce a meaningful amount of electricity (100 MWhr), a typical OTEC system will require about 300 m3/s of both cold and surface water. To bring that amount of cold water would require a Cold Water Pipe (“CWP”) about 1,000 m long and 15-18 m in diameter. Other variations of the OTEC system, namely the Open Cycle and the Hybrid Cycle would also require similar sized CWP.
The length, diameter and weight of the CWP represent a major challenge. The wave induced heave, roll and pitch of the floating platform will be transmitted to the CWP. The connection of the CWP with the floating platform must be sufficiently flexible to withstand such erratic movements or the platform, the connection or the CWP could be damaged.
The heavier the CWP is, the stronger the support needs to be, and yet it has to provide flexibility to accommodate the pitch, roll and heave produced by the seas. Since both the platform and the CWP are massive structures, the connection needs to be: (i) flexible enough to react to the sea movements, and; (ii) strong enough to accommodate the stress caused by the movement of a moving platform against the inertia of the CWP.
A typical OTEC plant might require, in addition to the CWP, a discharge water pipe (the “DWP”) to discharge the spent cold and surface water at sufficient depth to avoid mixing with warm surface water. Mixing the warm and cold water discharges would require a DWP of about 80 m (the final length will depend on the relative flow rates of cold and surface water and the depth/water temperature profile at the selected site). There are many alternative ways of collecting surface water, but it might also be convenient to provide for a surface water pipe (“SWP”), especially if providing for such structure could provide some protection to the CWP. Although the length and weight of the DWP and SWP will be smaller than those of the CWP, they are still massive structures providing similar challenges as the CWP. The diameter of the SWP would be similar to the diameter of the CWP (15-18 m), while the DWP would be about 1.5 times the diameter of either the CWP or SWP.